A balance transfer card allows you to transfer a balance from one credit card to another. Balance transfer credit cards often come with 0% interest for a fixed term, for instance the first four months of you owning your credit card.
This means that you can transfer the amount you owe from another credit card (usually for a fee), and not pay interest on the balance for the 0% period. This can give you more time to pay off what you owe without your debt increasing. When your 0% period comes to an end, the card’s usual interest rate will apply to any remaining balance.